In uplink of 3GPP LTE (3rd Generation Partnership Project Long-term Evolution, hereinafter, simply referred to as “LTE”), a P-SRS (Periodic Sounding Reference Signal) is used for measuring reception quality between a base station and terminals (see NPL 1, for example). The P-SRS is mapped to an SRS resource and transmitted from the terminals to the base station. At this time, first, a higher layer indication for each cell (hereinafter, referred to as a “first per-cell indication”) is used to set an SRS resource candidate group (hereinafter, referred to as a “first SRS resource candidate group”) common to all the terminals located in a target cell, that is, an “SRS resource pattern.” Then, a higher layer indication for each terminal (hereinafter, referred to as a “per-terminal indication”) is used to assign a plurality of SRS resource candidates constituting the first SRS resource candidate group to terminals to which SRS resources are to be assigned. The terminals map P-SRSs to the assigned SRS resources (hereinafter, referred to as “per-terminal SRS resources”) to transmit to the base station. It should be noted that each SRS resource candidate is a last symbol in an “SRS transmission candidate subframe.” Because none of the terminals in the cell in which the above-described “first SRS resource candidate group” is set transmits data using the SRS resource candidates, collision between an SRS and data signal is prevented.
In LTE, “srs-SubframeConfig” or the like is defined as the first per-cell indication. By means of the srs-SubframeConfig number shown in FIG. 1 transmitted from the base station to the terminals, the base station instructs the terminals on a transmission interval (TSFC) for transmitting P-SRSs, and an offset amount (ΔSFC) for designating a subframe to be used for starting transmission of P-SRSs. For example, when the srs-subframeConfig number is 4, the SRS transmission candidate subframes are the second (=1+ΔSFC), the seventh (=1+ΔSFC+(TSFC*1)), . . . , and the (1+ΔSFC+(TSFC*n))-th subframes (see FIG. 2).
Meanwhile, in LTE-Advanced (Re1.11), a heterogeneous cell configuration (HetNet: Heterogeneous Network) in which a pico cell is disposed in a macro cell has been proposed, and improvement of capacity in this heterogeneous cell configuration is being studied (see FIG. 3). In a heterogeneous cell, a signal transmitted from a terminal which belongs to the macro cell (hereinafter, referred to as a “macro cell terminal”) to a base station providing the macro cell (hereinafter, referred to as a “macro cell base station”) also arrives at a base station providing the pico cell (hereinafter, referred to as a “pico cell base station”). Therefore, interference between the macro cell and the pico cell in the pico cell (hereinafter, referred to as “pico cell interference”) becomes large.
There are possible methods for reducing the pico cell interference as described below. First, interference between data signals (for example, between PDSCH (Physical Downlink Shared CHannel)) can be reduced by assigning different frequencies to the macro cell and the pico cell (i.e., frequency division) by PDCCH (Physical Downlink Control CHannel) signaling used in LTE. Further, interference between control signals (for example, between PUCCH (Physical Uplink Control CHannel)) can be reduced by assigning different frequencies to the macro cell and the pico cell (i.e., frequency division) by RRC signaling used in LTE. Still further, interference between SRSs can be avoided by assigning different time to the macro cell and the pico cell (i.e., time division) by the RRC signaling. While interference provided to the pico cell base station by a signal transmitted to the macro cell base station will be mainly described here, the same applies to interference provided to the macro cell base station by a signal transmitted to the pico cell base station.
Here, applying the above-described first per-cell indication makes it possible to reduce interference provided to an SRS transmitted to the pico cell base station by a data signal (or an SRS) transmitted to the macro cell base station.
Specifically, the srs-SubframeConfig number indicating the first SRS resource candidate group is indicated to the macro cell terminal by the first per-cell indication. The “first SRS resource candidate group” described here includes all the desired SRS resource candidates constituting a “desired SRS resource candidate group” in the macro cell and all the desired SRS resource candidates constituting a “desired SRS resource candidate group” in the pico cell. That is, the first SRS resource candidate group indicated to the macro cell terminal needs to include the “desired SRS resource candidate group” in the pico cell. The “desired SRS resource candidate group” described here refers to a subframe group necessary as SRS resource candidates in each of the macro cell and the pico cell.
For example, when SRSs are transmitted at an interval of 5 ms in both the macro cell and the pico cell, the srs-SubframeConfig number in the pico cell can be set to 4 (transmission interval TSFC=interval of 5 ms, offset amount ΔSFC=1). At this time, the srs-SubframeConfig number in the macro cell can be set to 7 (transmission interval TSFC=interval of 5 ms, offset amount ΔSFC=0 and 1) (see FIG. 4).
In this way, the SRS resource candidates in the pico cell also become the SRS resource candidates in the macro cell.
With the SRS resource candidate group in the macro cell and the SRS resource candidate group in the pico cell set in this way, the SRS resources for only pico cell or the SRS resources for only macro cell are set to each SRS resource candidate by the per-terminal indication. Accordingly, it is possible to prevent the SRS transmitted from the macro cell terminal from colliding with the signal transmitted from the pico cell terminal, so that it is possible to reduce pico cell interference.